In typical double-flow steam turbines, a flow of motive steam is provided through an opening in an outer casing to an inlet chamber in an inner casing whereupon the steam is directed onto a first pair of annular rows of stationary blades positioned on either side of the middle of the turbine A number of such rows of stationary blades are fixed to the inner casing through attachment, by any one of several known methods, to an outer blade ring, of which there are several types. The radially inner end of the stationary blades is often terminated in a circumferential inner support ring which may be attached to the blades or may be formed integrally therewith. A bearing mounted rotor, having a number of annular rows of blades disposed about the periphery of the rotor is positioned within the inner casing so that the rotor blades are cooperatively associated with the rows of stationary blades. As the motive steam flows and expands from the turbine middle outward, stationary blades serve to direct the motive steam in a desired flow path onto the rotor blades to motivate the rotor in a well known manner.
In the above described double-flow steam turbines, the steam entering the turbine is directed transverse to the rotor axis, i.e., radially inward. As the steam reaches the area of the rotor blades, it must be turned through 90.degree. and then redirected by means of a first stationary circumferential row of blades onto a first stage of rotating blades. The operation of this first stationary blade row is important to the efficiency of the turbine since its purpose is to direct the steam entering the inlet into a preferred flow path onto the first row of rotating blades. Because there is a stationary blade connected to the static structure of the turbine and which is positioned adjacent the rotating rotor, there is necessarily formed a gap between a radially inward end of the first stationary blade row and the adjacent rotor. In many such turbines, steam entering the inlet may bypass the blades on the first blade row and flow around the ends of these blades through the gap between the inner support ring and the adjacent rotor. Because this steam is not directed into the desired steam flow path, it does not enter the first rotating blade row at the preferred angle and thus does not efficiently transfer its energy to the rotating blade row.
At least one form of apparatus utilized to avoid the loss of efficiency by steam flowing around the first stationary blade row is shown in U.S. Pat. No. 4,826,395 issued May 2, 1989 to Groenendaal, Jr. and assigned to Westinghouse Electric Corporation. This patent describes a system for use with double-flow steam turbines of the type in which the first stationary blade row comprises individually mounted blades and does not have a radially inner ring supporting the blades although the blade ends are fixed to a shroud. The apparatus as described therein comprises a pair of annular bands which circumscribe the rotor about its centerline with one of the bands being connected to the first row of stationary blades on one side of the inlet and the other band being connected to the first row of stationary blades on an opposite side of the inlet. The connection to the blades is a fixed or hard connection which does not provide for any relative movement between the connected band and the associated blades. At the junction between the two bands, there is an overlapping arrangement with a resilient seal positioned in one of the bands in a location so as to provide a frictional engagement with the other of the bands. This double band seal arrangement prevents steam from leaking between the bands and prevents steam from bypassing the first blade rows by passing around the ends of the blade row diaphragms. While this arrangement is suitable for some of the double-flow low pressure steam turbines, it does not provide for differential radial expansion but only for axial expansion between the opposed blade rows and it further does not provide for a method of connecting the circumscribing seal bands to a blade row of the type diaphragm having a inner ring.
Consequently, there is a need for a double-flow steam turbine which includes structure for preventing the flow of steam from circumventing the desired flow path in a turbine of the type having blade row diaphragms with inner diaphragm rings.